In an inkjet apparatus, in order to realize a high quality recording, the ink dot diameter needs to be made small. As a method of reducing the recording dot diameter, it is conventionally known to utilize a “pull-push driving” system where a pressure chamber communicating to a nozzle opening is contracted after temporarily expanded. According to this system, the mass of each ink droplet can be reduced, and the recording dot diameter can be minified.
As the recording heads utilizing piezoelectric elements as pressure generation devices, there are a system of applying a vibration plate (for example, a laminated piezoelectric layer method and a deflection mode method), and a shear deformation system where a partition wall of a pressure chamber is shear deformed without using the vibration plate.
In the laminated piezoelectric layer method which changes the volume of the pressure chamber via the vibration plate, since the piezoelectric element is disposed outside the pressure chamber, the shape and size of the piezoelectric element is not so much restricted, and it is possible to generate high pressure by using a powerful piezoelectric element, thus this method is good at ejection capability and ejection control of the ink droplet. However, the structure of such an inkjet head becomes complicated, manufacturing of a large capacity head is difficult, and a head having about 100 channels may be a limit.
In contrast, since the head of shear deformation mode system has a simple structure where grooves are formed to be pressure chambers in a piezoelectric element, a large capacity head having several hundred channels is possible to be manufactured. However, especially in the cases where drive signals of a rectangular pressure wave are applied to the recording head of shear mode system, ejection of a minute droplet is difficult due to the influence of pressure wave vibration in the pressure chamber.
In Examined Japanese Patent Application Publication No. 4161631 (Patent Document 1) described is a method of forming a minute droplet by utilizing a head of the shear mode system, applying voltages to deform the pressure chamber in order of a first expansion, contraction and a second expansion, and by controlling a ratio of the voltages and a width of the contraction pulse. Wherein, a pulse width of the first expansion pulse is referred as t1, a pulse width of the contraction pulse as t2, and a pulse width of the second expansion pulse is referred as t3.
However in a case where the pressure chamber is driven with a contraction pulse width t2 as described in the above mentioned Patent Document 1, a pressure wave vibration which is generated at the edge portion of the drive pulse cannot be effectively canceled and residual vibration remains largely. Therefore, to execute high frequency drive in this state is difficult. Further, Patent Document 1 describes an example of applying a second contraction pulse is applied to cancel the residual vibration. However, by applying the second contraction pulse, the total waveform of the pulses becomes long, which leads to decrease of the drive frequency. Further, even in the case where t2+t3=AL (AL: half of the acoustic resonance period of the pressure chamber) is satisfied without applying the second contraction pulse, as described in Patent Document 1, the residual vibration cannot be sufficiently canceled, which leads to greatly decreasing the drive stability. In order to obtain sufficient drive stability, it is necessary to wait for a sufficient time period until the residual vibration decays before the next drive, which results in the decrease of drive frequency.
Further, according to Patent Document 1, the droplet volume may be reduced to be 10 pl, however further reduction of the droplet volume is required in market.